your welcome. It might have a piece of dirt in the inside of the cap where the hole is. When heat builds pressure in the tank, if the hole is not open good, it will not vent right and cause fuel to get pushed out from the pressure.

My Exmark does the same if the fuel tanks are over 3/4 full. But the odor is strong, and I hate when it does that. Great question! I have wondered about the fuel filter too, but it still continued after I replaced the filter. I will get a couple of gas caps and try it.